The present invention relates to certain polymeric materials, and compositions containing them, which may be useful as charge transport materials. The invention also relates to processes for making these polymers and their use in devices such as electroreprographic devices and electroluminescent devices.
Polymers of the invention may be particularly useful in the field of electroreprography. Electroreprography is any process in which an image is reproduced by means of electricity and incident radiation, usually, electromagnetic radiation, more usually visible light. Electroreprography includes the technology of electrophotography which encompasses photocopying and laser printing technologies. Typically, in both a photocopier and a laser printer, a photo-conductive member is first charged in the dark (e.g. by applying a high voltage via a Corona discharge). Then a latent electrostatic image in charge is produced by partial exposure of the charged photo-conductive member (e.g. a drum or belt) to radiation (e.g. light). The radiation neutralises the charge to the exposed regions. The light source can either be reflected light from an illuminated image (photocopying) or from a laser which scans the photo-conductive member usually under instruction from a computer (laser printing). Once a latent image has been produced in charge, it is developed with toner, the toner is transferred onto a substrate (e.g. paper) and then fixed thereto (e.g. by heat) so that a visible image is obtained.
The photo-conductive member typically comprises a photo-conductor (e.g. an organic photo-conductor [xe2x80x9cOPCxe2x80x9d]) which must perform two different functions: generate a charge on exposure to the incident radiation; and transport the photo-generated charge to the surface. The unexposed regions of the photo-conductive member will retain their charge and form the latent image. It is usual to use different materials for each of these two processes and develop materials which are separately optimized for their ability to generate photo-induced charge (charge generating materials or xe2x80x9cCGMsxe2x80x9d) or their ability to transport charge (charge transport materials or xe2x80x9cCTMsxe2x80x9d). One aspect of the present invention is concerned with improvements in the field of CTMs.
The photo-conductor can be constructed as a single law or from a plurality of layers, for example from at least one charge generating layer (xe2x80x9cCGLxe2x80x9d) comprising the CGM and at least one separate charge transport layer (xe2x80x9cCTLxe2x80x9d) comprising the CTM.
An ideal photoconductor would be one where the material charges rapidly to a high value in the dark, retains the charge in the dark (i.e. exhibits no dark decay) and shows rapid total discharge on exposure to low-intensity illumination. The time taken for the charge-discharge cycle of a photo-conductor limits the maximum speed at which the latent image can be generated. Photo-conductive materials with improved electrical properties allow faster printing and copying.
The present invention relates to certain polymeric materials which may comprise triarylamine repeat units and which can offer improved properties as charge transport materials. Triarylamines are well known small molecule CTMs. Certain large molecule compounds and polymeric materials that comprise triarylamine moieties and/or repeat units are also known in the prior art, as described below.
DE 3610649 (BASF) discloses polymers of formula: 
where xe2x80x2n is from 1 to 100, and xe2x80x2X is H or Br. These polymers are made from an Ullmann coupling of tri- and/or di-bromotriphenylamine monomers and are not end capped (i.e. are not treated with a material with acts as an end capping reagent positively to control the molecular weight of the chains during polymerisation). This reference only suggests the use of these polymers as effective electrical conductors if doped either chemically (e.g. with tris-p-bromophonylaminiumhexachloroantimonate) or electrochemically (e.g. by anodic oxidation with conducting salt anions). This acts as a disincentive for a reader of this document to use undoped triarylamine polymers as CTMs in electroreprography, particularly as this field of use is not mentioned in this patent. This document does not suggest that it might be desirable to control the properties of these polymers during polymerisation, or how this might be achieved.
EP 0669654-A (Toyo ink) (=U.S. Pat. No. 5,681,664) discloses a hole transport material which is a copolymer of formula:
Hxe2x80x94Axe2x80x3xe2x80x94[Bxe2x80x3xe2x80x94Axe2x80x3]nxe2x80x94Bxe2x80x3xe2x80x94Axe2x80x3xe2x80x94H
where Axe2x80x3 is a aromatic amine moiety which may be a triarylamine and Bxe2x80x3 is a C4-7alicyclic moiety which optionally may contain heteroatoms. This document teaches that these copolymers need the alicyclic moiety Bxe2x80x3 to be an effective hole transport material and this would discourage a reader of this document from using polymers without this moiety as CTMs. These polymers are not intentionally end capped.
EP 0786106-A (Toyo ink) discloses light emitting compounds of formula: 
where each of A1xe2x88x92 to A4xe2x88x92 is a substituted or unsubstituted aryl group having 6 to 16 carbon atoms, and each of R1xe2x88x92 to RBxe2x88x92 is independently a hydrogen atom, a halogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryl group or a substituted or unsubstituted amino group, provided that adjacent substituents may form an aryl ring. These compounds are not polymers and there is no suggestion from this document to use polymers as CTMs.
EP 0827367-A (Xerox) disclose the use in electroluminescent (EL) devices of polynuclear amines of formula: 
where xe2x80x2R1 to xe2x80x2R5 are aryl groups and xe2x80x2A1 and xe2x80x2A2 are biaryl groups. These compounds are monodisperse molecules which are prepared by direct synthesis (e.g. Ullmann coupling), not by polymerisation. These compounds are not polymers. Indeed this patent teaches explicitly that polymeric CTMs are disadvantageous compared to the above compounds, as Xerox state that, unlike polymers, these compounds can be used to prepare a CTL by vapour deposition (see page 2, lines 29 to 31).
JP-A-08(96)-040995, 040996 and 040997 (all Togo ink) are consecutively numbered patent publications each of which discloses certain compounds which comprise triphenylamine residues. The compounds in stated to have utility in OLEDs and electrophotosensors. These triarylamine derivatives are molecular compounds and are not end gapped polymeric materials.
JP-A-08(96)-259938 (Togo ink) describes hole transport materials (for use in electrophotography and OLEDs) which are compounds of the formula: 
where: xe2x80x2xe2x80x3R1 to xe2x80x2xe2x80x3R14 are H, halogen, optionally substituted alkyl, optionally substituted alkoxy, optionally substituted thioalkoxy, cyano, amino, mono- or di-substituted amino, hydroxy, mercapto, optionally substituted aryloxy, optionally substituted arylthio, optionally substituted carbocyclic aromatic ring group, optionally substituted heterocyclic aromatic ring group, optionally substituted heterocyclic group with neighbouring substituents optionally forming optionally substituted alicyclic ring, optionally substituted carbocyclic aromatic ring, optionally substituted heterocyclic aromatic ring, optionally substituted heterocyclic ring: and xe2x80x2xe2x80x3n is 2 to 7. These molecules contain a saturated alicyclic or heterocyclic moiety, are not polymers and are not end capped.
U.S. Pat. No. 3,256,486 (Eastman Kodak) discloses doped linear polymers comprising triarylamine repeat units which have utility as photo-conductors in electrophotography where the polymer would perform the function of both the COM and CTM. This teaches away from the use of undoped linear polymers as a separate CTM in conjunction with a (different) CGM. The polymers disclosed are not end capped polymers and there is no suggestion that it would be desirable to control polymerisation or how this might be achieved.
U.S. Pat. No. 4,322,487 (Eastman Kodak): and Research Disclosure 19014 (Feb 1980); disclose particles for use in electrophoretic migration imaging. The particles comprise a colorant in a polymeric binder which further comprises triarylamine repeat units (the aryl groups being optionally substituted). These polymers are not end capped and are not used as CTMs.
U.S. Pat. No. 4,565,860 (Nissan) discloses a polymer comprising -[N(p-Ph)a]- repeat units (where Ph means para-phenylene or phenylenyl). The polymer is not end capped and is doped with an electron acceptor to be an effective electro-conductor. This teaches away from using either undoped or end capped triarylamine polymers as CTMs.
U.S. Pat. No. 4,741,603 (Nissan) discloses conjugated triphenylamine polymers which are used as the active component of an electrochromic mirror. These polymers are not end capped and are not designed for use as CTMs.
U.S. Pat. No. 4,801,185 (Nissan) discloses an electrochromic cell which comprises certain triphenylamine polymers. These polymers are not end capped and are also not designed for use as CTMs.
U.S. Pat. No. 5,476,740 (Xerox) describes a particular OPC device comprising CGMs and CTMs. One of the four preferred types of CTM listed (see col. 10, lines 66) is xe2x80x9cpoly triarylaminesxe2x80x9d. There is no further detail given of which triarylamine polymers are meant. End capped triarylamine polymers are not disclosed.
U.S. 5,877,096 (Ricoh) relates to a particular construction of OPC with TIOPc as the CGM. One of 26 CTMs types listed as usable in this OPC (see col. 10, lines 12 to 27) is xe2x80x9ctriarylamine derivativesxe2x80x9d. This general reference to triarylamine CTMs does provide any motivation to make end capped triarylamine polymers.
WO 97-33193 (Dow Chemical co.) discloses certain cross-linkable and chain extendable polyarylamines with utility in organic light-emitting materials (OLEMs). The polymers disclosed in this document are copolymers which achieve their stated cross-linkability and chain extending properties by comprising at least one reactive group selected from: hydroxy, glycidyl ether, acrylate ester, methacrylate ester, ethenyl, ethynyl vinylbenzoxyl, maleimide, nadimide, trifluorovinyl, ether, a cyclobutene attached to adjacent atoms on the aromatic group, and trialkylsiloxy. Dow argue that the properties required for a polymer to be a good CTM for are OLEM are different from those required for an electroreprographic CTM (see page 1, lines 14 to 29). There is no disclosure in this reference of how to prepare polymers using are end capping reagent to control molecular weight and no teaching that it might be advantageous to do so.
WO 98-06773 (Dow Chemical Co.) discloses certain polyarylamines with utility in OLEMs. The polymers have the formula: 
wherein:
Rxe2x88x92 is independently in each occurrence a C1-24hydrocarbyl, C1-24hydrocarboxy, C1-24hydrocarbylthiooxy, or C1-24hydrocarbylcarboxyl;
Arxe2x88x921 and Arxe2x88x922 are independently in each occurrence a C6-18 aryl moiety optionally substituted with a C1-12hydrocarbyl, C1-12hydrocarbyloxy, C1-12hydrocarbylthiooxy, or C1-12hydrocarbylcarboxyl;
Axe2x88x92 is independently in each occurrence hydrogen or a halogen;
xxe2x88x92 is independently in each occurrence a positive number from 0 to 1:
nxe2x88x92 is independently in each occurrence a whole number of from 0 to 4; and
mxe2x88x92 is a number from 5 to 1000.
These polymers are terminated with an Axe2x88x92 group which is either H or halo; comprise diamine repeat unit(s); and are not end capped. The method of polymerisation used to prepare these polymers does not readily control their polydispersity (i.e. mxe2x88x92 falls within a large of numbers). Thus it is difficult to optimise the properties of these polymers. As in the previous reference, Dow argue that CTMs designed for use with OLEMs are not necessarily good as electroreprographic CTMs (see page 1, lines 10 to 20).
WO 98/02018 (Bayer) discloses a particular construction of OLEM device comprising as the CTM a compound of formula: 
where xe2x80x3R2 is H, optionally substituted alkyl or halogen; and xe2x80x3R8 and/or xe2x80x3R4 may be (amongst other things) optionally substituted aryl. These large molecules comprise three triarylamine units (e.g. three triphenylamines) attached to a central benzine ring. These compounds are not polymers, are monodisperse and do not comprise intentional end capping groups. They are produced by direct chemical synthesis not polymerisation.
Synthetic Metals, 1991, Vol. 40, pages 231-238 (Nissan) discloses a method for the synthesis of certain polymers comprising -[N(p-Ph)3]- repeat unit. The polymers are described as electrical conductors when doped with, iodine but insulators when undoped. These are not end capped.
Macromal. Chem., 1992, Vol. 193, cages 909-919, xe2x80x9cThe higher homologues of triphenylamine: model compounds for poly(N-phenyl-1.4-phenyleneamine)xe2x80x9d discloses compounds of formulae: 
and 
Preparation of poly(N-phenyl-1,4-phenyleneamine), its dimer, trimer and tetramer are described. These compounds are monodiasperse, small molecule oligomers of up to 4 repeat units, and are not end capped. They are prepared by stepwise synthesis, not polymerisation,
Chem. Commun., 1997, page 2083 (Tanaka et al.) and Chemistry and Industry, Nov, 17, 1997, page 914; both disclose that certain branched bromo containing triphenylamine polymers may be useful CTMs. Tanaka et al state that a disadvantage of their preparation method is the lack of control over molecular weight. These polymers are not end capped.
Polym. Prep. (Am. Chem. Soc. Div. Polym. Chem.) 1997, Vol. 38(1), pages 388-389; Chem. Commun., 1986; pages 2175-2176; and Appl. Phys. Letts., Apr. 14, 1997, Vol. 70(15), pages 1829-1931 (all Toyota); disclose molecular hole transporting materials for use in an electroluminescent device. The materials have the following formulae: 
These molecules are prepared directly via an expensive multi-stage chemical synthesis which produces each molecule in a chemically pure monodisperse form. These molecules are not end capped with terminal groups and there would be no reason to do so as they are not produced by polymerisation. These materials ate monodisperse and consist of molecules of a single molecular weight. This is very different from a polydisperse polymeric material made try a polymerisation method which comprises a mixture of different polymeric species of varying chain lengths and with a distribution of molecular weights. The molecular weight for a polydisperse polymer would be calculated as an average value for the bulk polymer.
A paper by Kocheleva, Tameev et al. (from resp. Karpov inert. of Phys. Chem and A. N. Framing Inst. of Electrochem. of Rus. Acad. Sci.) was inducted in the proceedings from ISandT NIP 14:1998 International Conference on Digital Printing Techniques. Oct. 18 to 23, 1988, entitled xe2x80x9cCatalytic dehalogenation polymerisation of 4,4-dihalogentriphenylamines in the presence of a nickel complexxe2x80x9d (pages 528 to 531). This paper discloses triphenylamine oligomers comprising from 4 to 10 repeat units synthesised by nickel promoted dehalogenation polymerisation of 4,4xe2x80x2-dihalogentriphenylamines. These materials are not end capped or produced using an end capping reagent. They were tested as the CTM in an otherwise conventional dual layer photo-receptor. The paper explicitly teaches (see Table 5 below) that:
xe2x80x9celectrophotographic characteristics of oligomeric TPA improve ith increasing their molecular weights. Thus the photosensitivity of PTPA-3 is almost comparable to those of DEH end TPD.xe2x80x9d (page 530, col. 2 lines 1 to 4- underlining added).
Furthermore the paper states that the TPA oligomers:
xe2x80x9cdoped polycarbonate. . . [to show]. . . xerographic properties which were comparable to those for DEH end TPD.xe2x80x9d (page 531, col, lines 16 to 18xe2x80x94underlining added).
The results obtained in this paper were set out in Table 1 (col. 2, page 530) as follows:
DEH is (diethylamino)benzaldehyde diphenylhydrazone. TPD is bis(N,Nxe2x80x2-3-methylphenyl)bis(N,Nxe2x80x2-phenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine. Both DEH and TPD are well-known small molecule CTMs. PTPA-1, PTPA-2 and PTPA-3 ae various non end capped, triarylamine oligomeric CTMs that were made by nickel promoted dehalogenation polymerisation of 4,4xe2x80x2-halogentriphenylalamines as described in this paper. The photo-conductor tested was of a conventional dual layer construction (a CTL on a CGL). The CTL consisted of the same polymer binder (Mw=30,000) doped in each case with one of the CTMs listed above at a 1:1 weight ratio with the CTL binder. The CGL used in each test was a 2:1 respective ratio of TIOPc dispersed in a polyvinyl butyral binder. These results show that oligomeric CTMs made as described in this paper (without end capping) when tested at the same concentration exhibit only comparable photo-sensitivities to well known small molecule CTMs (such as TPD which is a small molecule triarylamine). There is no teaching in this paper which would suggest to a reader how the oligomers disclosed therein might be modified to exhibit much improved electroreprographic properties over the prior art.
Thus the prior art materials described above have various deficiencies as CTMs. For example the prior art teaches the use of large molecules comprising a triarylamine repeat unit in conductive layers, which must then be doped with additional materials (e.g. iodine) and/or must comprise additional substituents (e.g. bromo) to achieve good electrical conduction and/or further cross-linking or chain extension of the CTM. The prior art also teaches that triarylamine oligomeric CTMs are of low molecular weight, are produced by direct synthesis not polymerisation and/or are monodisperse. There is no teaching in the prior art of how polymerisation might be readily controlled to produce a satisfactory triarylamine polymeric CTM. There is empirical evidence in the prior art that triarylamine polymers exhibit only similar photosensitivity to small-molecule triarylamine CTMs.
The CTMs currently available are not completely satisfactory in some or all respects such as those discussed previously. Thus it would be desirable to provide CTMs which result in improvements in some or all of the aforementioned areas.
The applicant has unexpectedly discovered that certain end capped polymers, which can be based on triaryl amine repeat units(s), act as much improved charge transfer materials. This finding is in direct contradiction to what might have been predicted from the prior art. Thus it is very surprising that the end upped polymers of the invention overcome some or all of the aforementioned disadvantages with known CTMs.
Therefore broadly in accordance with the present invention there is provided a polymeric material comprising at least one repeat unit, the or each of more than one) repeat unit consisting substantially of a moiety of Formula 1: 
in which:
Y1 represents, independently if in different repeat unit, N, P, s, As and/or Se preferably N:
Ar1 and Ar2 which may be the same or different, represent, independently if in different repeat units, a multivalent (preferably bivalent) aromatic group (preferably mononuclear but optionally polynuclear) optionally substituted by at least one optionally substituted C1-40carbyl-derived groups and/or at least one other optional substituent and
Ar3 represents, independently if in different repeat units, a mono or multivalent (preferably bivalent) aromatic group (preferably mononuclear but optionally polynuclear) optionally substituted by at least one: optionally substituted C1-40carbyl-derived group and/or at least one other optional substituent,
where at least one terminal group is attached in the polymer to the Ar1, Ar2 and optionally Ar3 groups located at the end of the polymer chains, so as to cap the polymer chains and prevent further polymer growth, and at least one terminal group is derived from at least one end capping reagent used in the polymerisation to form said polymeric material to control the molecular weight thereof.
It will be appreciated that when the central atom (e.g. Y1 and/or Y2) in the repeat units of polymers of the invention and polymer precursors used to form them (denoted by the various formulae herein) is other than trivalent (e.g. divalent S and/or divalent Se), the number of aromatic groups (e.g. denoted by Ar1, Ar2 and for Ar3) attached thereto will be adjusted to correspond to the valence of the central atom (e.g. for divalent S and/or divalent Se, in Formulae 1 and/or 2, Ar3 and the arrow therefrom are not present and the repeat unit is divalent rather than optionally trivalent).
The number of the repeat units of Formula 1 which may be present in a particular polymer molecule of the invention (and which can also be denoted by the integer xe2x80x98nxe2x80x99 herein) may be from 2 to 20,000 inclusive.
The polymeric materials of the present invention are obtainable (preferably are obtained) by polymerisation controlled by addition of at least one end capping reagent in an amount sufficient to reduce substantially, further growth of the polymer chain.
The asterisks extending from Ar1 and Ar3 in Formula 1 are intended to indicate that these groups may be multivalent (including divalent as shown 1n Formula 1).
The arrows extending from certain polymers and moieties therein (for example from Ar3 in Formulas 1 and 2 herein and the ring to which R3 may be attached in Formula 3 herein) are intended to indicate that these groups may be monovalent or multivalent if these groups are monovalent the arrow denotes a bond to a suitable terminal group such as hydrogen or another substituent which is inert to coupling under the conditions of polymerisation (e.g. alkyl or aryl). In Formulae 2 and 3 hereinafter such a terminal group is denoted by R3 which is only present when the aryl group to which it is attached is monovalent. If the group is multivalent (e.g. bivalent) the arrow denotes a bond to another repeat unit (i.e. the polymer chain is branched and/or cross-linked).
The end capped polymers of the invention can be produced more cheaply and with a better control over their resultant properties (such as their molecular weight and polydispersity) due to the end capping. Furthermore the chemical nature of the end cap can to selected to control aspects of the polymerisation and hence properties of the resultant polymer. For example carrier mobility, polymer compatibility, electronic configuration [e.g. frontier orbital (FO) energy levels] and/or solubility may be strongly affected by substitution (if used) and/or molecular weight (e.g. mobility can be shown to increase with polymer molecular weight). The polymers of the present invention may act as very good CTMs, compared to similar polymers in the prior art. Thus it is surprising that end capped triarylamine polymers of the present invention, which can be easily prepared with controllable properties, may also be very effective, better CTMs compared to the prior art, as well as possessing other useful advantages.
The novel polymers of the present invention are of use as very effective CTMs in electroreprographic devices. However such polymers may have many other uses which may rely on the same, similar and/or different properties to those required for electroreprography.
For example the polymers of the present invention may be generally relevant for use in (and/or in combination with) any application and/or device which requires the use of polymeric conductors, polymeric photo-conductors, organic photo-conductors (OPCs), electroluminescent (EL) materials, polymeric materials which exhibit substantial conjugation over the polymer and/or polymeric semiconductors. Preferred polymeric semiconductors have hole mobilities greater than 0.01 cm2/volt sec. This minimum mobility is either that of the pure polymeric material, or of an admixture of the polymeric material with one or more other polymeric or monomeric materials having different electrical and/or physical properties. Preferably the polymers of the present invention also exhibit some or all of the following other useful properties: high carrier mobility, compatibility with binders, improved solubility, high durability and/or high resistivity undoped.
Preferably the polymers of the invention may be used in at least one of the following devices and/or for at least one of the following applications: electroreprographic devices (such as those described herein); electroluminescent (EL) devices {such as organic light emitting devices (OLEDs) [e.g, devices where the OLEM comprises a light emitting polymer (LEP)] and/or devices which comprise light emitting diodes (LEDs), [where the light emitting material may be inorganic but is preferably an organic, oligomeric or polymeric material]}; semi-conductor devices; photoconductive diodes; metal-semiconductor junctions (e.g. Schottky barrier diodes); p-n junction diodes; solar cells and/or batteries; photovoltaic devices (e.g. photovoltaic cells); photodetectors, optical sensors; phototransducers; bipolar junction transistors (BJTs), heterojunction bipolar translators and/or other switching transistors; field effect transistors (FETs) (which may comprises metal-semiconductor FETs, metal-insulator-semiconductor FETs and/or organic FETs); charge transfer devices (which may comprise charge coupled devices [CCDs]); lasers (which may comprise semiconductor and/or organic lasers); p-n-p-n switching devices (which may comprise semiconductor controlled rectifiers [SCRs]); optically active EL devices (which for example may be prepared by control of homochiral monomer polymerisation to achieve polarised light output, e.g, for 3-D imaging); thin film transistors (TFT, e.g. polymeric TFTs); organic radiation detectors; infra-red emitters; tunable microcavities for variable output wavelength; telecommunications devices and applications (for example a combination of OLEM, fibre optic and detector): optical computing devices (especially using materials with improved switching speeds): optical memory devices (for example devices which rely on external stimulus to trigger EL emission for devices run just below threshold onset voltage); general design of detectors and/or sensors (for example by combining EL excitation just below onset voltage, relying on external stimulation to trigger EL emission); chemical detectors (e.g. by combining EL with know or future luminescence detector systems); and combinations of any such devices and/or applications in which they are used.
In such applications and devices the polymer of the invention may be used either as the pure polymeric material, or of an admixture of the polymeric material with one or more other polymeric or monomeric materials having different electrical and/or physical properties. It may be laid down in a film form, (often less than one micron thick or even less than 250 nanometers thick) which can be optionally patterned or structured by a variety of coating or printing techniques such as dip coating, roller coating reverse roll coating, bar coating, spin coating, gravure coating, lithographic coating (including photolithographic processes), ink jet coating (including continuous and drop-on-demand, and fired by piezo or thermal processes), screen coating, spray coating and web coating. In the fully functional application or devices, the polymer of the invention or an admixture of polymer of the invention with one or more other polymeric or monomeric materials having different electrical and/or physical properties, may be in contact with metallic or non-metallic materials (having conducting, semi-conducting or non-conducting properties) In order to give a functioning application and/or device.
Certain of than applications require the tuning of the properties of the polymers of the invention which can readily be achieved by the preparation methods described herein, such as end capping. It will be understood that preferred polymers may have different, even opposite, optimal properties that those which are preferred and/or exemplified herein for electroreprographic applications. For example polymeric CTMs of the invention when optimised for use with organic light emitting materials [OLEMs] preferably may have a higher molecular weight and/or different mobilities than optimal for electroreprography.
Furthermore the compositions and/or specific polymers used for each application may be different. For example it is desirable that an electroreprographic polymeric CTM Is compatible with the binder polymers (such as polycarbonates) used to make a CTL By comparison a polymeric CTM for use in an OLEM may be formulated without many other (or wart no other) ingredients to make a film of substantially pure CTM. Thus each of these CTM polymers may require different physical properties.
More preferably polymers of the present invention are useful as charge transport materials (CTMs), most preferably in the fields of electroreprography and/or electroluminescent devices, especially electroreprography.
As mentioned above Ar1, Ar2 and Ar3 are each an optionally substituted aromatic group which may be a mononuclear aromatic group or a polynuclear aromatic group. A mononuclear aromatic group has only one aromatic ring, for example phenyl or phenylene. A polynuclear aromatic group has two or shore aromatic rings which may be fused (for example napthyl or naphthylene), individually covalently linked (for example biphenyl and/or a combination of both fused and individually linked aromatic rings. Preferably each Ar1, Ar2 and Ar3 is an aromatic group which is substantially conjugated over substantially the whole group.
Polymers of the present invention are end capped, that is polymerisation is controlled by adding at least one end capping reagent to limit further growth of the polymer chain. If the end capping reagent is added in excess (e.g. at the step when it is desired to terminate polymerisation) further growth of the polymer chain (and/or polymer network if the polymer is branched and/or moss-linked) can be substantially inhibited (e.g. substantially quenched). The end capping reagent adds terminal group(s) to the polymer chain which are substantially incapable under the conditions of polymerisation of undergoing coupling (e.g. with other polymer precursor and/or other parts on the polymer chain). The terminal groups) end cap the polymer chain end act to substantially reduce the possibility of (preferably stop) hither polymerisation by blocking sites at which the polymer chain could otherwise grow under the conditions of the polymerisation. Preferably in the polymers of the present invention from about 60% to substantially all of the polymerisation sites are blocked by at least one terminal substituent. More preferably (in one option) substantially all such sites are blocked. In another more preferable option from about 60% to about 90% of these sites are blocked.
Optional features of polymers of the invention which may further distinguish them from known polymers are any one or more of the following: invention polymers can be electroreprographically effective; invention polymers can have an Mn value of at least about 1000 daltons; invention polymers can comprise terminal group(s) other than those formed from bromobenzocyclobutene; invention polymers can comprise terminal group(s) other than a group selected from: (H, halo, hydroxy, glycidyl ether, acrylate ester, methacrylate ester, ethenyl, ethynyl, vinylbenzoxyl, maleimide, nadimide, trifluorovinyl, ether, a cyclobutene, a group forming part of a cyclobutene group, and trialkylsiloxy); invention polymers can be other than copolymer(s) which consist of triarylamine repeat unit(s) and C4-7alicyclic repeat unit(s) optionally containing heteroatom(s); invention polymers can be substantially undoped: and/or invention polymers can be substantially polydisperse.
Preferably the reagents to be reacted to form a polymer of the present invention, comprise a polymer precursor (normally considered as a monomer, although it could also be for example a polymerisable low molecular weight oligomer, such as a dimer or trimer) which is capable of being polymerised to form a polymer of the invention together with at least one end capping reagent.
Preferably the polymers of the invention comprise at least 3, more preferably at least 4, most preferably at least 6, repeat units of Formula 1 or Formulae 2 or 3 hereinafter.
Preferably the terminal group(s) comprise at least one group derivable from (more preferably derived from) an and capping reagent selected from: at least one; optionally substituted C1-40carbyl-derived molecule.
Preferred polymeric material of the present invention comprises a substance represented by Formula 2: 
where Ar1, Ar2, Ar3 and Y1 represent, independently in each case, those atoms) and/or group(s) as described herein;
n represents an integer from 3 to about 500;
R1, R2 and R3 represent, independently, a terminal group as described herein, R3 only being present when Ar3 is not attached to another repeat unit.
Preferably in Formulae 1 and/or 2, Ar1, Ar2 and Ar3 are each independently an optimally substituted, aromatic carbyl-derived group, more preferably an optionally substituted heterocyclic and/or benzenoid ring which comprises an aromatic group, most preferably the optionally substituted aromatic group is, or forms part of, a bivalent C6-40hydrocarbyl, and especially is selected from phenylene and naphthenyl (both optionally substituted, preferably try C1-15alkyl).
More preferred polymeric material of the present invention comprises a substance represented by Formula 3: 
where R1, R2, R3 and n represent, independently if in different repeat units, those groups or values described herein, R3 only being present when the ring to which it is attached is not itself attached to another repeat unit;
a and b represent, independently in each case, 0 or an integer from 1 to 4;
c represents, independently in each case, 0 or an integer from 1 to d (where d is 5 minus the valence of the aromatic group), preferably 0 to 5;
n represents an integer from d to about 200; and
R4, R5 and R6 represent, independently in each case, optionally substituted C1-15alkyl and/or at least one optional substituent.
As written above the substance which is represented by Formulae 2 and/or 3 will be a single polymer molecule in which the integer xe2x80x98nxe2x80x99 denotes the number of repeat units in an arbitrary one of the many chains which may comprise a bulk polymer. It will be understood that the integer xe2x80x98nxe2x80x99 in Formulae 2 and 3 could be replaced by the real number xe2x80x98mxe2x80x99 [which is an average for xe2x80x98nxe2x80x99 aver the whole polymer]In which can the substance represented by Formulae 2 and 3 would be a bulk polymer, rather than one of the molecular chains which comprise such a polymer. In such a substitution the values given for xe2x80x98nxe2x80x99 in Formulae 2 and 3 may remain unchanged for xe2x80x98mxe2x80x99, except that non-integral values would be allowed for xe2x80x98mxe2x80x99. The differences between xe2x80x98nxe2x80x99 and xe2x80x98mxe2x80x99 herein, and preferred values thereof, are discussed more fully hereinafter.
The optional substituents on the aromatic repeat unit(s) are those listed herein. Preferably they may be selected to improve the compatibility of the CTM with the binder resins in which they may be formulated to form the CTL. Thus, the size and length of the substituents may be selected to optimise the physical entanglement or interlocation of the polymeric CTM with the binder resin. The choice of substituent also effects electronic properties and hence mobility of charge carriers.
Preferably the terminal groups (which are attached to the repeat units of Formula 1 and denoted by R1, R2 and, if present R2, in Formulae 2 and 3) are unreactive groups, that is are substantially incapable of undergoing chain extension or cross-linking under the conditions of polymerisation. More preferably the terminal groups are independently affected from at tenet one optionally substituted C1-40hydrocarbyl group, most preferably selected from C1-30alkyl, C8030aryl and C7-30aralkyl, any of which frilly be optionally substituted. Especially preferred terminal groups comprise C6-36aryl optionally substituted with at least one: C1-4alkyl (itself optionally substituted by at least one halo); C1-4alkoxy (itself optionally substituted by at least one halo); amino (itself optionally N-substituted by at least one C1-4alkyl). In particular the terminal group may be selected from: phenyl optionally substituted with at least one methyl, 2-methylprop-2-yl, methoxy, ethoxy, trifluoromethyl and/or diethylamino.
Particular polymers of the invention may be formed from at least one specific monomer selected from:
bis(N-4-chlorophenyl)-3-methylphenylamine;
bis(N-4-chlorophenyl)-4-methylphenylamine;
bis(N-4-chlorophenyl)-2,4-dimethylphenylamine;
bis(N-4-chlorophenyl)-4-(Nxe2x88x92,Nxe2x88x92-diethyl)aminophenylamine;
bis(N-4-chlorophenyl)-3-trifluoromethylphenylamine;
bis (N-4-chlorophenyl)phenylamine;
bis (N-4-chlorophenyl)-2,5-dimethylphenylamine;
bis (N-4-chlorophenyl)-3-methoxyphenylamine;
bis (N-4-chlorophenyl)-4-ethoxyphenylamine;
bis (N-2-methyl-4-chlorophenyl)-2,4-dimethylphenylamine;
bis (N-4-chlorophenyl)-4-(2-methylprop-2-yl)phenylamine;
tris (N-4-chlorophenyl)amine; and/or
mixtures thereof.
Alternatively particular polymers of the invention may comprise at least one terminal group derived from at least one especially preferred and capping reagent selected from:
1-chloro-3-methylbenzene; 1-chloro-4-methylbenzene; 1-chloro-3-trifluoromethylbenzene; 1-chloro-3-methoxybenzene; 1-bromo-2,4-dimethylbenzene; (N-4-chlorophenyl)diphenyl amine; 1-bromo-4-(2-methylprop-2-yl)benzene; chlorobenzene; and/or mixtures thereof.
Specific polymeric materials of the invention may be those obtainable by the polymerisation of any combination and/or mixture of at least one especially preferred monomer (as described above) in the presence of, and/or which is substantially quenched by, at least one especially preferred end capping reagent (as described above).
There is empirical evidence in the prior art (see previously) that shows triarylamine polymers are not significantly more effective as CTMs compared to well-known small molecule triarylamines (e.g. TPD). It was believed that the mixtures of component polymer molecules of differing chain lengths in a polydisperse polymer would result in charge trapping and thus prevent rapid hole transport within a polymeric CTM.
Yet because of the control of polymerisation by end capping, the applicant has shown herein that, very surprisingly, the end capped triarylamine polymers of the present invention can exhibit significantly improved performance as CTMs in an electroreprographic device.
Without wishing to be bound by any theory it is believed that in a transport material, charge carriers (e.g. positive holes) move via a series of oxidation-reduction steps from one molecule to another (so-called xe2x80x9chopping charge transportxe2x80x9d). It is thought that the highest energy electron of the molecule is delocalised over a substantial part of the molecule. Thus enlarging the size of the conjugated xcfx80 system world increase the probability of electron transfer. Preferred polymers of the present invention may achieve this by substantially complete conjugation throughout the whole length of the polymer chain and/or polymer network (If the polymers are cross linked).
However polymers of the invention may also comprise oligomeric conjugated sections (e.g. of repeat unit of Formula 1) stitched together with non-conjugated sections (such as aliphatic sections, for example xe2x80x9cWxe2x80x9d orientated n-propyl groups) to produce polymers of the present invention which are incompletely conjugated but which have optimal merge transfer characteristics between the conjugated sections. Such polymers may be co-polymers prepared using a difunctional non-corrugated, co-polymer linking agent (such as 1,3-dichloropropane). During preparation of these co-polymers this linking agent may also act as an end capping reagent.
For efficient and rapid transport of charge, polymer molecules preferably do not contain areas where charge carriers may be localised (trapped). The nature of the end capping reagent may therefore influence the degree to which charge carriers may be trapped (if at all) on a polymer molecule. By choosing appropriate terminal groups on a polymer molecule of the present invention, charge mobility can be advantageously optimised (e.g. in electroreprography, residual image can be eliminated).
It is also believed that frontier orbital energy level(s) in the polymers of the present invention may be tuned to match them with the energy levels of other materials with which a polymer of the invention may be required to interact. Such tuning might be achieved for example by varying electron density using suitable substituents and/or by varying conjugation length by adjusting the values of xe2x80x98nxe2x80x99. Varying the polydispersity of the polymers of the invention may also influence the range of frontier orbital (FO) energy levels and this may afford the opportunity of selecting ranges of FO energy levels to provide a staircase of energy levels between different materials. By this means the properties of polymers of the invention may be optimised to form, where desirable, an electrical bridge between particular materials with which they may be used.
The use of end capping reagents to control polymerisation allows facile preparation of polydisperse polymers which can be very effective CTMs. This has many further advantages. Polydisperse polymers are straightforward to manufacture using polymerisation techniques which are much cheaper than direct chemical synthesis of a large molecule. By comparison monodisperse, large molecules would be very difficult to produce by polymerisation methods as isolation of a component of a single chain length from the polymer mixture would be very onerous and expensive it not practically impossible. With end capping it is feasible to prepare, as effective CTMs, polydisperse polymers of long chain lengths and high molecular weight, which could not readily be made by direct synthesis. CTMs which have a wide variety of desired and/or optimised properties can thus be prepared. The properties of polydisperse polymers can also be readily modified by altering the composition of polymer precursor(s) [normally monomer(s)] used in their preparation [e.g, by using a mixture of selected monomer(s)] and/or altering the composition of polymer(s) in the resultant CTM [e.g. by using a mixture of selected polymer(s)].
The end capped poetic CTMs of the present invention have improved compatibility with the diluents (e.g. resin binders) used in the CTL. For example the polymers of the invention do not lower the Tg or polymeric diluents to which they are added to the same degree as prior art small molecule CTMs. This is in direct contrast to known CTMs which are generally small molecules which tend to lower resin Tg and hence durability to a greater extent. Thus optionally when electroreprographically effective amounts of a CTM of the present invention are used to make a CTL the formulation retains a high Tg which gives the CTL improved durability. The speed of an electroreprographic device depends on the concentration of CTM within the CTL, whereas its lifetime is a function of its durability. Therefore devices made using a CTM of the present invention may ether have a much increased life for a given speed or be faster for a given lifetime. It is also desirable to have materials of high Tg for use in electroluminescent devices.
Polymeric CTMs with branched and/or cross-linked structures may be used to further improve structural strength and durability of the CTL.
A further advantage of the polymeric CTMs of the present invention is there is a much reduced tendency for them to crystallise within the CTL. Such crystallisation would lead to CTL failure. Thus if necessary higher concentrations of these CTMs can be safely incorporated (e.g. dissolved) in a CTL without re-crystallisation problems. A higher loading of CTM allows even faster transport of charge within the CTL and hence faster electroreprographic devices.
A polymer of the invention may be substantially wholly linear in structure; or may have a degree of chain branching. In the latter case the degree of chain branching may be sufficient for the polymer to be cross-linked, the polymer is other than a substantially wholly linear polymer, i.e. if it is branched or even cross-linked, the polymer must comprise at least one aromatic repeat unit which is tri- or higher valent, (for example generally Ar3 in Formula 1 is not monovalent but multivalent, e.g. bivalent) so there is a moiety capable of providing chain branching and even cross-linking the resulting polymer.
Optionally polymers of the invention have a branched polymer structure which may even form a cross-linked network of polymer chains through direct bonds to the Ar1, Ar2 and Ar3 groups. Such branched and/or cross-linked polymers may provide improved structural strength and durability. If polymers of the present invention are branched the polymer may comprise xe2x80x98side chainsxe2x80x99 attached to a main chain. There are many ways a branched polymer can be arranged, for example xe2x80x9cstar-branchingxe2x80x9d. Star branching results when a polymerisation starts with a single monomer and has branches radially outward from this point. Polymers with a high degree of brunching are called dandrimers. Often in these molecules, branches themselves have branches. This tends to give the molecule an overall spherical shape in three dimensions.
Branching in the polymer chain may be introduced by addition of a tri (or higher) functional monomer, preferably a triarylamine substituted by at least three coupling groups (such any of those defined herein e.g. for X1 to X4 hereinafter); more preferably a triphenylamine substituted by three or more groups selected from any of chloro, bromo and/or iodo; most preferably tris(chlorophenyl)amine.
Nevertheless polymers of the invention also include those where polymer chains are substantially linear (e.g. comprise mainly di-substituted repeat units where the monomers attach via the Ar1 and Ar2 groups); those which are substantially branched (e.g. having a significant proportion of tri-substituted repeat units where the monomers attach via the Ar3 group as well as the Ar1 and Ar2); those which are combined (e.g. comprise any suitable proportion of regions with linear and regions with branched repeat units within the same polymer chain); and/or any suitable mixtures of such polymer chains. The degree of blanching of a polymer of the invention may be defined by the ratio of bivalent to monovalent Ar3 groups within the polymer. This ratio is preferably within the range from about 0 (linear) to about 1.0, more preferably from about 0.001 to about 0.5 of bivalent Ar3 to monovalent Ar3 respectively (i.e. a high number signifies more branching).
In an entirely optional aspect of the invention it may be desirable that a polymer of the invention does not consist of only linear polymers (i.e. the polymer may comprise [even in a trace amount] at least one polymer molecule which is not solely linear, such as a polymer molecule which comprises a branched region). Nevertheless preferred polymers of the invention are those which are substantially linear, including those which are wholly linear.
The polymers of the invention have s controlled polydispersity, i.e. the length distribution of the different polymer gains can be controlled. The length of each polymer chain within a polydisperse polymer mixture corresponding to an independent value of the integer xe2x80x98nxe2x80x99 herein (e.g. as denoted in Formulae 2 and 3), and these may be readily controlled by end capping the chains during polymerisation. Preferred polymer chains are those with values of n from 3 to about 500, more preferably from 4 to about 200, most preferably from 6 to about 50 and especially from 8 to about 30. A polymer of the present invention may consist substantially of polymer chains with the aforementioned xe2x80x98nxe2x80x99 values.
In the polymers of the present invention a value may be determined by known methods for the average number of repeat units per chain over the whole bulk polymer. This average value is denoted herein by the real number xe2x80x98mxe2x80x99 which, as an average and/or calculated value, need not be an integer. It will be understood that xe2x80x98mxe2x80x99 is distinct from the specific value for a particular polymer chain denoted by the integer xe2x80x98nxe2x80x99 herein (e.g. in Formulae 2 and 3), Preferred polymers exhibit an xe2x80x98mxe2x80x99 value of from about 3 to about 200, more preferably from about 4 to about 100, most preferably from about 4 to about 50. It is advantageous that xe2x80x98mxe2x80x99 is from about 6 to about 40, more advantageously from about 6 to about 20, for example from about 8 to about 14. Preferably the polymers of the invention comprise a mixture of polymer chains with a substantially Gaussian distribution of chain lengths, although other distributions are also possible [such as non-symmetrical (e.g. skewed) and multi-modal (e.g. bimodal) distributions].
The process developed by the applicant for polymerising the polymer precursor(s) in the presence of an end capping reagent (i.e. a chain terminator) produces the end coped polymers of the invention, and enables one to more ready control the polydispersity of the resultant polymer. Preferably the polymers of the invention have a polydispersity (Mw/Mn) from about 1.1 to about 5.0. More preferably from about 1.1 to about 3.0 (where Mw denotes weight average molecular weight and Mn denotes number average molecular weight). Polydispersity may be measured by any convenient method (such as gel permeation chromatographyxe2x80x94GPC).
Preferably the polymer of the invention has an Mn value of from about 700 daltons to about 120,000 daltons more preferably from about 700 to about 50,000 daltons; most preferably from about 1000 daltons to about 40,000 daltons. Advantageously invention polymers have an Mn of from about 1,100 to about 15,000 daltons, more advantageously from about 1,500 daltons to about 12,000 daltons, in particular from about 1,800 daltons to about 8,000 daltons. The preferred method for measuring and/or determining Mn values herein is from gel permeation chromatography (xe2x80x9cGPCxe2x80x9d) using a mufti angle laser light scattering (MALLS) detector and/or a refractive indent (RI) detector.
Polymers that comprise substantially one type of monomer in the chain are called xe2x80x98horrhopolymersxe2x80x99 whereas polymers which incorporate more than one kind of monomer into their chain are called xe2x80x98copolymersxe2x80x99. Copolymers may comprise the random, block or graft type. A random copolymer contains chains which have a random arrangement of the multiple monomers. A block copolymer contains chains with blocks of monomers of the same type. A graft copolymer contains chains which have a backbone comprising one type of monomer with pendant branches made up of at least one type of monomer. Polymers may also form bonds between neighbouring chains. These bonds can be formed directly between the neighbouring chains, or two chains may bond to a third common molecule. Preferred polymers of the invention comprise all polymeric forms that are electroreprographically effective.
Preferably the polymers are substantially free of repeat units other than those of the formulae given herein, and a given polymer of the invention preferably has only one repeat unit. Where a polymer has only one repeat unit it is likely to be a homopolymer, although in principle it is possible to employ a polymer precursor made from two or more different monomers so that a polymer derived from that precursor alone would have only one repeat unit yet be a copolymer. If a polymer of the invention has more than one repeat units (i.e. two or more repeat unit; which are different from each other) it is likely to be a copolymer.
The polymers of the invention may be electroreprographically effective without being doped with species such as halogen radicals or halide ions. Thus preferably the polymers of the invention may be 95% free (by weight) of such impurities. Apart from halo species, other dopants in the polymer which readily form spades (such as anlons) which can trap positive holes may in fact reduce ability of the polymer to transport positive charge from the CGL. Thus it is preferred, although not essential, that the polymers of the invention are substantially free of impurities.
The polymers of the present invention may be coloured or substantially colourless.
When in the formulae herein there is a list of labels (e.g. Ar1 and Ar2) or indices (e.g. xe2x80x98nxe2x80x99) which are said to represent a list of labels or numerical values, and these are said to be xe2x80x9cindependent in each case xe2x80x9d this indicates each label and/or index can represent any of than groups listed, independently from each other, independently within each repeat unit. Independently within each Formula and/or independently on each group which is substituted; as appropriate. Thus in each of these instances many different groups might be represented by a single label (e.g. Ar1).
The terms xe2x80x98optional subslituentxe2x80x99 and/or xe2x80x98optionally subsbtutedxe2x80x99 as used herein (unless followed by a list of other substituents) signifies at least one of the following groups (or substitution by these groups): sulfo, formyl, amino, imino, nitrilo, mercapto, cyano, nitro, halo, C1-4alkyl. C1-4alkoxy, hydroxy and/or combinations thereof. These optional groups may comprise all chemically possible combinations in the same group and/or a plurality (preferably two) of the aforementioned groups (e.g. amino and sulfonyl if directly attached to each other represent a sulfamoyl radical). Preferred optional substituents comprise: any of C1-4alkyl, methoxy and/or ethoxy (any of these optionally substituted by at least one halo); and/or amino (optionally substituted by at least one methyl and/or ethyl); and/or halo.
The term xe2x80x98carbyl-derivedxe2x80x99 as used herein denotes any monovalent or multivalent organic radical moiety which comprises at least one carbon atom either without any non-carbon atoms (e.g. xe2x80x94Cxe2x89xa1Cxe2x80x94), or optionally combined with et least one other non-carbon atom (e.g. alkyl, carbonyl etc.). The non-carbon atom(s) may comprise any elements other than carbon (including any chemically possible mixtures or combinations thereof) that together with carbon can comprise an organic radical moiety. Preferably the non-carbon atom is selected from at least one hydrogen and/or heteroatom, more preferably from at least one; hydrogen, phosphorus, halo, nitrogen, oxygen and/or sulfur, most preferably from at least one hydrogen, nitrogen, oxygen and/or sulfur. Carbyl-derived groups include all chemically possible combinations in the same group of a plurality (preferably two) of the aforementioned carbon and/or non-carbon atom containing moieties (e.g. alkoxy and carbonyl if directly attached to each other represent an alkoxycarbonyl radical).
The term xe2x80x98hydrocarbylxe2x80x99 as used herein (which is encompassed by the term xe2x80x98carbyl-derivedxe2x80x99) denotes any radial moiety which consists only of at least one hydrogen atom and at least one carbon atom. A hydrocarbyl group may however be optionally substituted.
Preferably xe2x80x98carbyl-derivedxe2x80x99 moieties comprise at least one of the following carbon containing moieties: alkyl, alkoxy, alkanoyl, carboxy, carbonyl, formyl and/or combinations thereof; optionally in combination with at least one of the following heteroatom containing moieties: oxy, thio, sulfinyl, sulfonyl, amino, imino, nitrilo and/or combinations thereof.
More preferred carbyl-derived groups comprise at least one: alkyl and/or alkoxy (optionally substituted with at least one halo).
The term xe2x80x98alkylxe2x80x99 or its equivalent (e.g. xe2x80x98alkxe2x80x99) as used herein may be readily replaced, where appropriate, by terms denoting a different degree of saturation and/or valence e.g. moieties that comprise double bonds, triple bonds, and/or aromatic moieties (e.g. alkenyl, alkynyl and/or aryl) as well as multivalent species attached to two or more substituents (such as alkylene).
The term xe2x80x98haloxe2x80x99 as used herein signifies fluoro, chloro, bromo and iodo.
Any radical group or moiety mentioned herein (e.g. as a substituent) refers to a monovalent radical unless otherwise stated or the context clearly indicates otherwise (e.g. an alkylene moiety is bivalent and links two other moieties). Unless the context clearly indicates otherwise, a group herein which comprises a chain of three or more atoms signifies a group in which the chain wholly or in part may be linear, branched and/or form a ring (including spiro and/or fused rings). The total number of certain atoms is specified for certain substituents for example C1-xcex1-hydrocarbyl, signifies a hydrocarbyl moiety comprising from 1 to xcex1 carbon atoms. In any of the formulae herein if at least one ring substituents are not indicated as attached to any particular atom on the ring, the substituent may replace any H attached to an atom in the ring and may be located at any available position on the ring which is chemically possible.
In an optional proviso, the carbyl-derived groups and/or the optional substituents herein may comprise and/or be other than: hydroxy, glycidyl ether, acrylate ester, methacrylate ester, ethenyl, ethynyl, vinylbenzoxyl, maleimide, nadimide, trifluorovinyl ether, cyclobutene or a group forming part of a cyclobutene group, and trialkylsiloxy.
Unless the context clearly indicates otherwise, as used herein plural forms of the terms herein are to be construed as including the singular form and vice versa.
The term xe2x80x98electroreprographically effectivexe2x80x99 (for example with reference to the polymers of the present invention) will be understood to comprise those ingredients which if used in electroreprography in the correct manner provide the required properties to the composition and/or device necessary to generate a charge on exposure to incident radiation, transport said charge and/or to form an image therefrom; and which are compatible with the diluent(s) used to formulate electroreprographic compositions. Preferred xe2x80x98electroreprographically effectivexe2x80x99 materials (especially for CTMs) are those which are capable of supporting the injection of photo-generated charge (e.g. positive holes) from a CGM and/or CGL and/or are capable of allowing the transport of charge (e.g. positive holes) through the CTM and/or CTL. xe2x80x98Electroreprographically inertxe2x80x99 refers to a material which would not be xe2x80x98electroreprographically effectivexe2x80x99 as defined herein and/or would not substantially adversely effect electroreprographic performance.
Where the context indicates, the term xe2x80x9celectroreprographically effectivexe2x80x9d may be replaced by another term such as xe2x80x9ceffective for usexe2x80x9d as it will be understood that polymers of the invention may also be of use in fields other than electroreprography, such as any of those mentioned herein.
It is believed that the polymers of the invention are highly electroreprographically effective, exhibiting excellent properties as CTMs. However should any of the polymers claimed herein should be found not to be electroreprographically effective they still form part of the present invention. Such polymers would have utility as intermediates in the preparation of more electroreprographically effective polymers, as tools to investigate the mode of action of the polymeric CTMs of the invention, and/or in the other non electroreprographic uses described herein.
Certain polymers of the invention and/or moieties therein (such as repeat units), may exist in many different forms for example at least one form from the following non-exhaustive list: isomers, stereoisomers, enantiomers, diastereoisomers, geometric isomers, tautomers, conformers, forms with regio-isomeric substitution, isotopically substituted forms, polymeric configurations, tactic forms, interstitial forms, complexes, chelates, ciathrates, interstitial compounds, non-stoichiometric complexes, stoichiometric complexes, ligand complexes, organometallic complexes, solvates and/or mixtures thereof. The present invention preferably comprises all such forms of the polymers of the invention, moieties therein, any compatible mixtures thereof and/or any combinations thereof, preferably those which are electroreprographically effective.
At least one polymer of the present invention has utility as charge transfer material (CTM) for use in electroreprography and/or electroluminescence (EL). For example the combination of the CTM polymers of the present invention with an EL material (such as an LEP), whether in multi-layer or admixture, can improve EL efficiency as the energy levels of the polymeric CTM can be tuned (e.g. as described herein) to best match the other materials in the electrical current chain. However, a preferred use of the polymers of the present invention is in electroreprography.
Therefore in a further aspect of the invention there is provided a composition suitable for use as a charge transport material (CTM), optionally for use in an electroreprographic device, the CTM comprising (optionally in a substantially pure form) at least one polymeric material of the present invention as described herein; optionally together with a substantially electroreprographically-inert diluent. Preferably the diluent comprises polymer(s) other than those of the present invention.
It will be appreciated that compositions comprising polymers of the invention may be formulated differently for according to the end use, with different amounts of polymer and/or different additional ingredients. For example for use with OLEMs a preferred composition may provide a film which comprises mostly (preferably at least about 50%, more preferably at least about 80%) the polymers of the invention. Such a film may most preferably comprise substantially 100% polymer of the invention. On the other hand electroreprographic formulations may comprise less than these amounts of polymers of the invention and are preferably formulated as described below.
Polymers of the present invention may be used in combination with any diluent (which preferably comprises at least one resin binder), CGM, other CTM and/or any other ingredient conventionally used in electroreprography to formulate an electroreprographically effective composition (e.g. a CTL and/or CGL). Optionally the composition may be formulated for use in a single layer electroreprographic device (where the CTM and CGM are in the same layer). However formulations optimised for dual and multi layer devices are preferred (i.e. where there is at least one CTL and at least one separate CGL). Preferred electroreprographic compositions are those which can be used to form the or a CTL on a substrate in a suitable part of the electroreprographic device (e.g. on top of the CGL on a photoreceptor drum) and/or which may form the CTL directly. The CTL may be formed by any suitable method (e.g. spin coating, vapour phase deposition and/or immersion of the substrate in a liquid composition).
Compositions which are used to prepare a CTL may additionally comprise a solvent so they can be applied to the substrate (e.g. the CGL) as a liquid, the CTL being formed by evaporation of solvent. Suitable solvents may be any solvent commonly used in photoreceptor manufacturing, preferably selected from at least one of: toluene, tetrahydrofuran (THF), ethyl acetate, chlorobenzene, dichloromethane, dichloroethane, n-butyl acetate and/or mixtures thereof.
Liquid compositions used to prepare a CTL of the invention may comprise the solvent in an amount from about to from about 50% to about 99%, more preferably from about 60% to about 95%, most preferably from about 70% to about 90% based on the mass of the total liquid composition. The remainder of the liquid composition may comprise those ingredients described below for the CTL in the relative proportions described therein.
The diluent may comprise any substantially electroreprographically inert material, preferably a binder resin, more preferably a resin which is a good electrical insulator. The binder resin is preferably selected from at least one: polyamide, polyurethane, polyether, polyester, epoxy resin, polyketone, polycarbonate [e.g. poly(4,4xe2x80x2-isopropylidine-diphenylene carbonate (such as those available commercially from: GEC under the trade name Lexan, from Bayer under the trade name Makrolon and/or from Mobay Chem. Co. under the trade name Merion). PCA, PCZ and/or co-polymers of polycarbonates [e.g. those copolycarbonates described in JP-A-07(95)-271061 and 271062 (both Fuji-Xerox)}], polysulfone, vinyl polymer (for example polyvinylketone and/or polyvinylbutyral [e.g. PVB]), polystyrene, polyacrylamide, copolymers thereof (such as aromatic copolymeric polycarbonate polyesters [e.g. those available commercially from Bayer under the trade name APEC]) and/or mixtures thereof. Preferred binder polymers are those having molecular weights (Mn) from about 20,000 to about 120,000 daltons, more preferably from about 50,000 to about 100,000 daltons.
PCA denotes bis-phenol-A polycarbonate resin.
PCZ denotes poly(4,4xe2x80x2-cyclohexylidenediphenylene carbonate) resins comprising a repeat unit of formula: 
PCZ is available commercially (directly or indirectly) from Tejin for example under the trade name Panalite.
The diluent may optionally further comprise at least one plasticiser, which is preferably selected from at least one: halogenated paraffin, polybiphenyl chloride, dimethylnaphthalene, dibutyl phthalate and mixtures thereof.
The diluent may be selected for its hardness and durability. However polymers of the invention with a suitable high value for average molecular weight may also be sufficiently durable to be used without a diluent. For example polymers of the invention that may be used without a diluent, may be those polymers with a Tg comparable to those of conventional resin binders which are used as substrates for CTLs (e.g. polycarbonate and/or polyester resins). Alternatively a CTL composition may comprise a mixture of polymers of the invention with diluent resins, and it is advantageous if the mixture has a Tg comparable to conventional resin substrates used to make CTLs. Preferably a CTL composition of the invention has a Tg which is within about 50xc2x0 C. of the Tg of that composition when substantially free of CTM (and which may correspond to the Tg of its component diluent resin(s) when substantially pure). Such compositions can be particularly durable when used in an electroreprographic device. The Tg is measured by the known method of differential scanning calorimetry (DSC).
The CGMs which may be used in conjugation with the CTMs of the present invention may be any known in the art, as well as any new CGMs which may be discovered in the future and which would be readily apparent to those skilled in the art to be suitable for use with the polymeric CTMs of the present invention.
Thus, for example, suitable CGMs may comprise an inorganic photoconductor (which may be crystalline or a glass), an organic photoconductor and/or an charge transfer complex.
Preferably the CGM may be selected from at least one of the following materials: inorganic photoconductive substances such as: inorganic crystalline materials (for example compounds such as zinc oxide, zinc sulfide, cadmium sulfoselenide, cadmium selenide, cadmium sulfide and mixtures thereof, materials such as trigonal selenium, and mixtures of any of these compounds and materials); inorganic glasses (for example amorphous selenium, vitreous selenium, and/or selenium alloys [e.g. Se/Te, Se/Te/As, Se/As and/or mixtures thereof]);
substituted and unsubstituted metallo- or metal-free phthalocyanine [Pc] compounds (for example, metal free phthalocyanines [such as H2Pc], metal phthalocyanines, [such as Cu, Ni, Mg, Zn or Co Pc], titanyl phthalocyanine [TiOPc], vanadyl phthalocyanine [VOPc], and/or other phthalocyanines [e.g. InClPc, AlClPc, AlClPcCl, t-Bu1.4VOPc and/or GaOHPc]); naphthalocyanines;
aquarylium compounds (e.g. squaraines and/or squariliums);
azuleniums;
azo compounds (e.g. azo pigments);
perylene compounds (e.g. perylene pigments, perylene tetracarboxydilmide and/or bis-imidazole perylene [BZP]);
indigo compounds (e.g. indigo pigments);
quinacridones;
polycyclic quinones (for example polynuclear aromatic quinones such as anthraquines and/or anthanthrones [e.g. dibromo anthanthrone known herein as xe2x80x9cDBAxe2x80x9d])
cyanine compounds (e.g. cyanine dyes);
xanthene compounds (e.g. xanthene dyes);
thiapyriliums (for example their salts);
diamino triazines (for example substituted 2,4 diamino triazines);
triphenodioxazines;
3,6-diphenylpyrrolo[3,4-c]pyrrole-1,4-dithione
charge transfer complexes comprising an electron donor, e.g. poly-N-vinylcarbazole and an acceptor e.g. trinitrofluorenone;
eutectic complexes formed by pyrylium salts (e.g. dyes) and polycarbonate resins; and/or any mixtures thereof.
More preferably the CGM comprises phthalocyanines (for example metal free phthalocyanine, H2Pc, TlOPc, GaOHPc and/or VOPc); perylenes (e.g. BZP) and/or polycyclic quinones (e.g. DBA).
Most preferably the CGM comprises metal-free Pc and/or TiOPc in any electroreprographically effective, polymorphic form which is now known or is discovered in the future. Known polymorphs of metal free Pc include the X form (XPc) and the tau form (xcfx84Pc). Known TiOPc polymorphs include those denoted as types I (xe2x89xa1xcex2), II (xe2x89xa1xcex1), III (xe2x89xa1m), IV (xe2x89xa1Y or xcex3), X, Z and Za [e.g. as described in U.S. Pat. No. 5,189,156 (Xerox) and GB 2322866-A (Zeneca)]. Particularly preferred CGMs are selected from at least one: TiOPc(I), TiOPc(Za) and TiOPc(IV) polymorph.
Preferably the optional other CTM(s) in a composition of the invention may be selected from at least one of the following materials capable of transporting charge (which are preferably non-polymeric): triarylamine; hydrazone; triphenylmethane, oxazole, oxadiazole; styrilic; stilbene, butadiene and/or any combinations thereof (including combinations of these functional moieties in the same molecule) and/or mixtures thereof.
More preferably additional CTMs may comprise tetrakis(N,N-aryl)biaryldiamines, most preferably bis(N,Nxe2x80x2-[substituted]phenyl)bis(N,Nxe2x80x2-phenyl)-1,1-biphenyl-4,4-diamines, especially the 4-methyl, 2,4-dimethyl and/or 3-methyl derivatives thereof.
Preferred electroreprographic compositions of the invention, which may be used to prepare a CTL in an electroreprographic device and/or which may form such a CTL, comprise from about 8% to about 100%, more preferably from about 10% to about 75%, most preferably from about 15% to about 50% of polymer(s) of the present invention and from about 0% to about 92% more preferably from about 25% to about 90%, most preferably from about 50% to about 85% of electroreprographically inert diluent(s) (such as those described herein). All percentages are by mass of ingredient to the total mass of the composition.
Suitable photo-conductors with a CTM of the invention may be formed from a single OPC layer or from a plurality of CTL(s), CGL(s) and other layer(s) and can be fabricated as known to persons skilled in the art [for example as described in GB 1577237 (Xerox), especially FIGS. 1 to 4 therein]. The thickness of a CTL of the invention (including a single layer which combines the function of both a CTL and a CGL) may be from about 0.01 xcexcm to about 50 xcexcm, preferably from about 0.2 xcexcm to about 30 xcexcm. The thickness of a (separate) CGL which may be used in conjugation with a CTL of the present invention, may be from 0.01 xcexcm to about 20 xcexcm, preferably from about 0.05 xcexcm to about 5 xcexcm.
Other conventional aspects of OPC devices and compositions, including other binder(s), CGM(s), non-invention CTM(s), arrangements and/or optimal thicknesses of CTL(s) and/or CGL(s), may readily be included in and/or used with the CTMs and compositions of the present invention. Such details are known to persons skilled in the art of electroreprography and are disclosed in: xe2x80x9cChemistry and Technology of Printing and Imaging Systemsxe2x80x9d published by Blackie Academic and Professional (1996), edited by P. Gregory, (see especially Chapter 4, xe2x80x9cElectrophotographyxe2x80x9d); and the review paper xe2x80x9cOrganic Photoconductive Materials Recent Trends and Developmentsxe2x80x9d by K. Y. Law, Chem. Rev., 1993, Vol. 83, pages 449-86. The disclosures of both these documents are incorporated herein by reference. It will be appreciated that future developments in these areas (such as future OPC chemicals or device construction) could also be used in conjunction with the polymers of the present invention.
In a still further aspect of the present invention there is provided a method for making a composition of the present invention, by mixing at least one polymer of the present invention with at least one (optionally substantially electroreprographically inert) diluent.
The method may further comprise making a charge transport layer (CTL) by coating onto a substrate a composition and/or at least one polymer of the present invention.
The polymers of the present invention may be prepared from at least one suitable polymer precursor which may comprise any suitable (co)monomer(s), (co)polymer(s) [including homopolymer(s)], (co)oligomers [including homo-oligomers(s)], and mixtures thereof which comprise aromatic moieties which are capable of forming a bond with the or each polymer precursor(s) to provide chain extension and optional cross-linking with another of the or each polymer precursor(s) via direct bond(s) as indicated in the Formulae herein. The polymer precursor(s) may be substantially unreactive at normal temperatures and pressures. Polymerisation may be initiated by any suitable means which are well known to those skilled in the art for example: chemical initiation by adding suitable agents; catalysis; photochemical initiation using an initiator followed by irradiation at a suitable wavelength; and/or thermal initiation. After a suitable period an end capping reagent is added (preferably in excess) to quench polymerisation.
Therefore in another aspect of the present invention there is provided a process for making an optionally electroreprographically effective end capped polymeric material; the process comprising the steps of:
a) performing polymerisation of at least one polymer precursor (preferably at least one monomer) of Formula 4: 
in which:
Y2 independently represents, N, P, S, As and/or Se, preferably N;
Ar4, Ar5, and Ar6 which may be the same or different, each independently represent at least one mononuclear or polynuclear aromatic group optionally substituted by a substituent which does not react with other groups on the polymer precursor(s) under the conditions of polymerisation;
X1, and X2 which may be the same or different, each independently represent a leaving group which, under the conditions of polymerisation, permits coupling between the aromatic groups to which they are attached and an aromatic group not linked thereto, directly via a bond [this provides chain extension and optional cross-linking, optionally with another of the or each polymer precursor(s)]; and
X3 independently represents H, another group inert to coupling or a leaving group which, under the conditions of polymerisation, permits coupling between the aromatic group to which it is attached and an aromatic group not linked thereto, directly via a bond [this provides chain extension and optional cross-linking, optionally with another of the or each polymer precursor(s)]; and then:
b) adding an end capping reagent of Formula 5 in an amount sufficient to reduce substantially polymerisation (optionally the end capping reagent is added in excess), and where in Formula 5:
Txe2x80x94X4xe2x80x83xe2x80x83Formula 5
T represents H and/or a carbyl-derived radical, preferably H, C1-40hydrocarbyl and/or C5-40aryl, optionally substituted by at least one substituent which does not react with other groups on the polymer precursor(s) under the conditions of polymerisation; and
X4 represents at least one group in the compound of Formula 5 which, under the conditions of polymerisation, permits coupling between T and an aromatic group on the growing polymer directly via a bond, so as to end cap the chain and provide chain termination.
If X3 is H, or another group inert to coupling, then a linear polymer is formed. If X3 is a leaving group then a branched and/or cross-linked polymer is formed.
Preferably polymerisation (step xe2x80x98axe2x80x99) may be further controlled if carried out and/or started in the presence of a certain amount of the end capping reagent (chain terminator) of Formula 5, such that the reaction proceeds to the desired degree of polymerisation. It will be understood that the ratio of end capping reagent to polymer precursor will control the average degree of polymerisation. Preferably, if present, the respective mass ratio of polymer precursor to end capping reagent in step xe2x80x98axe2x80x99 is less than about 5; most preferably from about 0.001 to about 3; more preferably from about 0.01 to about 2 (see also the Examples herein).
The time interval between initiating polymerisation in step xe2x80x98axe2x80x99 and quenching polymerisation in step xe2x80x98bxe2x80x99 depends on the particular reagents used but may usefully be from about 30 minutes to about 100 hours, preferably from about 1 to about 25 hours more preferably from about 2 to about 10 hours (see also the Examples herein). The end capping reagent may be present during initiation of polymerisation and/or added in one or more suitable aliquots during polymerisation before the final quenching in step xe2x80x98bxe2x80x99. For example the end capping reagent can be added in third, quarter or half fractional amounts of the total to be added at suitable (e.g. hourly) intervals starting from (e.g.) about 1 to about 5 hours after initiation of step xe2x80x98axe2x80x99 (see also the Examples herein).
Preferably the respective mass ratio of the initial amount of polymer precursor polymerised in step xe2x80x98axe2x80x99 to the end capping reagent added to quench polymerisation in step xe2x80x98bxe2x80x99 is an excess, more preferably within the polymer precursor/end capping reagent ratios given previously (see also the Examples herein).
Preferably Ar4, Ar5, and Ar6 which (as shown) represent bivalent radicals may independently comprise the bivalent equivalent of the groups listed herein for the monovalent radicals Ar1, Ar2 and Ar3 respectively in Formulae 1, and/or 2 herein, while Ar6xe2x80x94X3 may also represent the monovalent equivalent as set out herein for the monovalent radical Ar3 in Formulae 1 and/or 2 herein.
Similarly T may comprise the monovalent equivalent of the bivalent radicals listed herein as Terminal Substituents and/or as R1 and R2 in Formulae 1, 2 and 3 herein.
A yet further aspect of the present invention is any polymer obtainable by the above process including all the electroreprographically effective different forms of such polymers and/or moieties therein.
It is understood if it is desired to make a polymer which is not linear, i.e. branched or even cross-linked, the polymer will comprise at least one aromatic/repeat unit which is trivalent, i.e. in the polymer precursor of Formula 4, X3 represents a group capable under the conditions of polymerisation of forming a bond in a coupling with another aromatic group to which it is not attached.
Preferably the polymerisation is carried out in the presence of a catalyst, more preferably the catalyst comprises nickel.
Preferably the chain terminator of Formula 5 is present in an amount of from about 1% to about 50%, more preferably from about 10% to about 20% w/w of the total amount of the at least one polymer precursor of Formula 6.
Preferably X1, X2, X3 and X4 are independently selected from at least one halo, more preferably fluoro, chloro and iodo, most preferably chloro. It is positively preferred that if at least one of X1 to X4 is bromo then at least one of X1 to X4 is a halo other than bromo.
The degree of polymerisation of polymers of the present invention can be controlled by the molar ratio of the polymer precursor(s) of Formula 4 to the chain terminator of Formula 5.
Preferably at least one polymer precursor of Formula 4 comprises at least one compound of Formula 6: 
where
R7, R8 and R9, independently in each case, comprise at least one group selected from H, optional substituents and optionally substituted C1-40carbyl-derived groups, and R7, R8 and R9 are incapable of reacting with other groups on the polymer precursor(s) under the conditions of polymerisation;
X1 to X2 independently comprise a suitable leaving group;
p, q and r independently represent 0 or 1, except at least two of them must be 1;
u, v and w independently represent 0 or an integer from 1 to 5, except at least two of them must be other than 5; and
(p+u); (q+v); and (r+w) are all from 0 to 5 or less; except at least three of them must be other than 0.
Preferably in Formula 6:
R7 to R9 comprise, independently in each case, at least one C1-15carbyl-derived group; and
X1 to X3 comprise, independently in each case: fluoro, chloro, bromo, iodo, optionally substituted arylsulfonyl, optionally substituted C1-8alkylsulfonyl and/or diazonium salt.
More preferably R7 to R9 may be, independently in each case: amino (optionally substituted by at least one C1-4alkyl); C1-4alkyl (optionally substituted by at least one halo) and/or C1-4alkoxy (optionally substituted by at least one halo);
More preferably X1 to X3 comprise, independently in each case: fluoro, chloro, bromo, iodo, optionally substituted phenylsulfonyl, optionally substituted C1-4alkylsulfonyl and/or diazonium salt (preferred optional substituents being at least one methyl, bromo, fluoro and/or nitro).
Most preferably X1 to X3 may be, independently in each case: chloro; bromo; 4-methylphenylsulfonyl; 4-bromophenylsulfonyl; 4-nitrophenylsulfonyl; methylsulfonyl; trifluoromethylsulfonyl; 2,2,2-trifluoroethylsulfonyl; 4-fluorophenylsulfonyl; 2-trifluoromethyl-1,1,1,3,3,3-hexafluoroprop-2-ylsulfonyl; and/or diazonium salt.
More preferred monomers of Formula 6 are those in which: p and q are both 1; r is 0 and/or 1; u, v and/or w are independently 0, 1 and/or 2; X1, X2 and X3 are Cl; R7 and R8 are both methyl; and R9 is independently selected from methyl; 2-methyl-prop-2yl, methoxy, ethoxy, trifluoromethyl and diethylamino.
Specific monomers that may be used in the above process are those listed previously and/or those which are used to prepare the Examples herein.
Preferably the end capping reagent of Formula 5 comprises at least one compound of Formula 7: 
in which,
R10 comprises, independently in each case, H, optionally substituted C1-40 carbyl-derived groups or at least one other optional substituent, and R10 is incapable of reacting with other groups on the polymer precursor(s) and/or growing polymer chain under the conditions of polymerisation;
a represents 0 or an integer from 1 to 5; and
X4 comprises a suitable leaving group.
Preferably R10 comprises any of those groups specified for R7, R8 and/or R9 previously.
Preferably X4 comprises any of those groups specified for X1, X2 and/or X3 previously.
Preferably at least one of the polymers and/or CTMs of the present invention (which may be obtainable by the process of the invention) comprises an end capped polymer of Formula 8: 
where
R7 to R10 and n, p, q, r and s represent, independently in each case, those groups and values given herein.
While Formula 8 as written above shows a linear polymer, which is preferred, an option within the scope of the present invention is for at least one aromatic ring to which R9 is attached not to be monovalent (as shown in Formula 8) but bivalent so that at least one polymer of Formula 8 is other than a linear polymer, i.e. is branched and/or cross-linked.
A yet still further aspect of the present invention provides an electroreprographic device, photo-conductive member for said device; component for said device and/or consumable for use with said device, which comprises at least one polymer and/or CTM composition of the present invention as described herein. The device may be selected from at least one: photocopier, printer, optionally laser printer, fax machine, scanner and multipurpose devices for copying, faxing and/or scanning. The photo-conductive member may be selected from a photosensitive drum and/or a photosensitive belt.
The electroreprographic device, photo-sensitive member, component, and/or consumable may be prepared by a method, comprising the steps of forming a charging generating layer (CGL) on a substrate; and then forming on the CGL a charge transport layer (CTL) comprising a composition and/or at least one polymer of the present invention.
Another aspect of the invention provides use of at least one polymer and/or CTM of the present invention in the operation and/or manufacture of a electroreprographic device, component for said device and/or consumable for use with said device, preferably for the purpose of transporting charge.
Polymers of the present invention may exhibit markedly improved electrical and/or mechanical performance as CTMs compared to the prior art CTMs.
Therefore a yet further aspect of the invention comprises using at least one polymer of the present invention in an electroreprographic composition, photosensitive member and/or electroreprographic device for the purpose of improving electroreprographic performance. Electroreprographic performance may be measured in many ways, for example by decay exposure, time of flight (TOF) and/or residual voltage, and can be compared to a similar electroreprographic composition, photosensitive member or electroreprographic device in which the end capped polymeric CTM of the invention is replaced by an substantially identical amount (in % w/w) of a known CTM (e.g. a well known small molecule triarylamine such as TPO).